Conventionally, in Patent Documents 1 and 2, there are disclosed organic EL devices each in a structure in which a light-emitting layer is formed directly on a hole transport layer or an anode. In the organic EL device, the hole transport layer is formed of a polymer thin film, and the light-emitting layer is formed of a polymer phosphor and an electron-donating and/or electron-accepting organic compound. Whereas, in Patent Document 3, there is disclosed a structure of lamination of a low-molecular-weight hole transport layer formed with a deposition method, and a polymer light-emitting layer formed with a coating method.
However, as in the case where an organic EL device is applied as an in-vehicle display device, when the organic EL device is used under severe environment, it is important that the hole transport layer is formed of low-molecular-weight material. However, it is not preferable that the hole transport layer is formed of a polymer thin film as with the structures described in Patent Documents 1 and 2. With the mechanism by which the polymer hole transport layer transports holes, the polymer material repeats an oxidation-reduction cycle, and thereby carries out hole transport. However, the oxidized polymer material is not necessarily returned to the original state, but may become another reaction product upon oxidation. This conceivably results in that the layer partially ceases to function as the hole transport layer, thereby to be deteriorated. Particularly, the polymer hole transport layer is considered to be more remarkably deteriorated than the low-molecular-weight hole transport material. Thus, the hole transport layer including a polymer material has a problem in durability.
Whereas, in Patent Document 3, the hole transport layer is formed of a low-molecular-weight material. For this reason, there is no problem in terms of durability. However, when the light-emitting layer is formed of a mere polymer material, the interface between the hole transport layer and the light-emitting layer is the interface between the low-molecular weight material and the polymer material. Accordingly, the differences in physical states such as film density at the interface unfavorably result in a low adhesion, and also a low carrier injectability.
Further, conventionally, there are a low-molecular-weight compound type organic EL device and a polymer type organic EL device. When a low-molecular-weight compound type organic EL is adopted, it is possible to form an organic EL device with a high high-temperature durability and a long lifetime; but, a large-scale apparatus including a large number of combined vacuum deposition devices (vacuum deposition chambers) is required, unfavorably resulting in a very high manufacturing cost. Whereas, when a polymer type organic EL device is adopted, the vacuum deposition step required during the manufacturing process is only a cathode formation step, and for other steps, a non-vacuum process can be used. This allows a reduction of the manufacturing cost. However, on the other hand, there are problems such as low high-temperature durability due to instability of the polymer hole transport layer including PEDOT: PSS, or the like, short lifetime, and voltage increase associated with driving.
For use as an in-vehicle display device, particularly a segment display device, there is required an organic EL device low in cost, high in high-temperature durability, and long in lifetime. For this reason, both the devices are difficult to use so long as they remain as they are.
For this reason, an organic EL device is under consideration which has solved the respective problems by adopting a multilayer structure of a low-molecular-weight hole transport layer formed with a vacuum deposition method and a polymer light-emitting layer formed with a coating method. However, when the low-molecular-weight hole transport layer and the polymer light-emitting layer are in contact with each other, the low-molecular-weight material forming the low-molecular-weight hole transport layer dissolves into the solvent of the polymer coating solution.
Thus, in Patent Document 3, there is proposed an organic EL device configured such that the low-molecular-weight material forming the low-molecular-weight layer is less likely to dissolve in the solvent of the polymer coating solution. Specifically, to the low-molecular-weight hole transport material, a crosslinkable organic compound having a siloxane skeleton, a crosslinkable organic compound including a silane coupling compound, or a crosslinkable organic compound including at least one of a double bond group, an epoxy group, and a cyclic ether group is mixed, and the resulting mixture is deposited. After deposition, the material is subjected to heat polymerization, photopolymerization, or electron beam polymerization, thereby to be made poorly soluble. As a result, the low-molecular-weight material becomes less likely to dissolve in the solvent of the polymer coating solution.
Whereas, as a technology of reducing the solubility in the solvent of an organic coating film, there is also known solubility reduction by photopolymerization using a photosensitive coating solution used in a photolithography method.
However, as shown in Patent Document 3, with the method Of solubility reduction by effecting heat polymerization, photopolymerization, or electron beam polymerization after deposition, or solubility reduction by photopolymerization using a photosensitive coating solution, a material containing an instable crosslinkable functional group is required to be used. For this reason, the device characteristics tend to become instable. Further, unpolymerized radicals remain in the film, which tends to result in a short lifetime. Further, for example, even when solubility reduction is accomplished, with the foregoing method, it is not that dissolution is completely inhibited. Accordingly, although improved, there still remains a problem that the constituent material of the low-molecular-weight hole transport layer dissolves into the solvent of the polymer coating solution.
Further, self light-emitting devices such as EL (electroluminescence) devices are capable of high-luminance light emission, and is capable of being reduced in power consumption and being reduced in thickness of the apparatus. Thus, the self light-emitting devices receive attention as next-generation display devices or light source devices. In an organic EL device which is one kind of the self light-emitting devices, for the light-emitting layer provided between the hole injection electrode and the electron injection electrode, an organic material is used. This results in high degree of freedom for the light emission color. On the other hand, there are problems such as low durability due to the low coverage resulting from the very small film thickness of the light-emitting layer or the like, and the low durability of the adopted organic material itself. Thus, there have been conducted research and development for enhancing the durability.
For such organic EL devices, there have been proposed a low-molecular-weight compound type organic EL device using a low-molecular-weight material as the organic luminescent material and a polymer type organic EL device using a polymer material. In the case of the low-molecular-weight compound type organic EL device, not only for the light-emitting layer but also for the hole transport layer or the electron transport layer, low-molecular-weight organic materials are respectively used. Respective layers can be sequentially stacked on a substrate with a vacuum deposition method or the like. Such a low-molecular-weight compound type organic EL device has already come into practical use. Thus, not only as individual low-molecular-weight materials used but also as the whole device, the reliability is being improved.
On the other hand, for the EL device using a polymer material as a luminescent material, as a method for stacking the polymer material on a substrate, there can be adopted a method of coating, printing, or the like. Whereas, a single polymer organic material often includes both of the light-emitting function and the carrier transport function. This makes it easy to obtain an organic EL device of a simple configuration in which a monolayer polymer organic material layer is formed between electrodes. Further, as described above, the polymer organic material can be formed with a printing method. For this reason, it is easy to pattern the polymer organic material on a per RGB pixels basis.
Further, there is also a trial to adopt both of the low-molecular-weight organic material and the polymer organic material for the organic EL device. For example, in Patent Document 4, there is shown an organic EL device configured as follows: on a substrate, a first electrode is formed; on the first electrode, a first organic film layer formed of a low-molecular-weight light-emitting material is formed; on the first organic film layer, a second organic film layer formed of a polymer electron transport material is formed; and on the second organic film layer, a second electrode is formed. For the organic EL device, not that the first and second organic film layers are sequentially stacked on the first electrode, but that the second organic film layer and the first organic film layer are provided sequentially from the film side on an additionally adopted donor film. From the donor film, the first organic film layer (low-molecular-weight light-emitting substance layer) and the second organic film layer (polymer electron transport substance) are simultaneously transferred onto the first electrode on the substrate with a laser induced thermal transfer imaging (LITI) method.
In Patent Document 5, there is proposed an organic EL device which includes functional layers including at least a light-emitting layer between a first electrode and a second electrode. The organic EL device includes, as the functional layers, from the first electrode side, a light-emitting layer using a polymer organic material, and on the light-emitting layer, an electron transport layer farmed of, a low-molecular-weight organic material. Further, it is proposed that a hole injection/transport layer using a polymer organic material as with the light-emitting layer is formed between the first electrode and the light-emitting layer. The hole injection/transport layer and the light-emitting layer using a polymer organic material are formed with an ink jet printing method.
In Patent Document 6, there is proposed an organic EL device including a hole injection/transport layer, and a light-emitting layer stacked from the anode side between the anode and the cathode. For the hole injection/transport layer, a polymer material is used, and the layer is formed with a wet method. For the light-emitting layer, a low-molecular-weight light-emitting material is used, and the layer is stacked using a wet method.
Whereas, Patent Document 7 describes as follows. Between a first electrode layer and a second electrode layer, a light-emitting medium lamination is included. The light-emitting medium lamination includes a layer formed as a low-molecular-weight material layer including a low-molecular-weight material having a weight-average molecular weight of 1000 or less, and a layer formed as a polymer material layer including a polymer material having a weight-average molecular weight of 1000 or more. Further, at least one low-molecular-weight material layer and at least one polymer material layer are stacked with the surfaces in contact with each other. In other words, the polymer material layers and the low-molecular-weight material layers are stacked alternately one on another. This configuration can inhibit the occurrences of crystallization and aggregation on the surface of the low-molecular-weight material layer.
As described above, the low-molecular-weight compound type organic EL device is capable of forming an organic EL device high in high-temperature durability and long in lifetime. However, deposition using a vacuum deposition method requires deposition in a vacuum chamber. When a different light-emitting layer is formed for each of RGB pixels, patterning using a deposition mask is required. Further, as a single low-molecular-weight organic material, a material having all the functions of hole transport, light emission, and electron transport has not yet been developed. Thus, multiple layers are required to be stacked to form the device. This requires a large-scale apparatus including a large number of combined vacuum deposition devices (vacuum deposition chambers). Accordingly, the manufacturing cost tends to be very high.
On the other hand, for the polymer type organic EL device, the vacuum deposition step is used only for a cathode formation step. For other steps, a non-vacuum process can be used. Therefore, the manufacturing cost can be reduced. Further, as described above, patterning is easy. Also in this respect, the polymer type organic EL device is advantageous in reduction of the manufacturing cost. However, the organic EL device using a polymer organic material has more problems in terms of low high-temperature durability, short lifetime, voltage increase associated with driving, and the like than the low-molecular-weight compound type organic EL device.
As in Patent Documents 4 to 7, for the organic EL device, both of a low-molecular-weight material and a polymer material are used. This creates the possibility of obtaining advantages of polymer/low-molecular-weight compound. However, in actuality, in aiming at practical implementation, there are many problems including problems in durability, stability, and the like, and problems in terms of control of voltage increase and the like. Particularly, the requirement in terms of durability, especially, the durability under high-temperature high-humidity environment cannot be satisfied even when the multilayer structure of polymer/low-molecular weight compound is merely adopted.